1. Field of the Invention
The invention relates to an F-theta objective, which is chromatically corrected for broadband laser radiation, for focusing a scanning high-power laser beam into a flat image field and has, arranged along the optical axis, lenses of different materials which are resistant at a laser output of more than 1 kW.
2. Description of the Background Art
In laser material processing, increasingly lasers having high outputs are used for welding or separating parts. One disadvantage of these lasers is their relatively great chromatic bandwidth and absorption of the laser radiation by the lenses of the F-theta objective and thus the heating thereof.
The chromatic bandwidth and heating influence the focusability of the laser radiation.
An F-theta objective commonly used for laser material processing is adapted for monochromatic radiation. The position of the focal plane and the geometry of the scan field are thus determined by exactly one wavelength. If chromatically broadband laser radiation is used, a separate focal plane for each wavelength and a scan geometry which differs according to each wavelength are obtained. The effect for laser material processing is a laser focus which is significantly greater, both in the spatial depth (longitudinally) and also orthogonally with respect thereto (laterally), than with use of monochromatic radiation. Thus, the processing surface area, which is subjected to laser radiation, on the material to be processed is larger, the power density correspondingly lower and the power necessary to achieve the desired processing effects higher.
Another effect when using F-theta objectives with high-power laser radiation is the heat that is introduced into the optical system. As a result, the position of the focal plane changes with the change in temperature of the F-theta objective. The distance between the F-theta objective and the processing plane, however, is generally kept constant, and therefore the focal point moves longitudinally with respect to the processing plane and the laser radiation is no longer focused in the processing plane, but instead a defocused beam spot is imaged here. A temperature-controlled tracking of the processing plane would be extremely complex. In terms of material processing, this means that the beam spot imaged on the processing plane is greater and thus the power density lower. In the worst case, no more material processing is performed because the power density has dropped below the processing threshold.
It is thus necessary to correct both the position of the focal plane and the scan geometry for the chromatic bandwidth. In addition, if possible no heat, or only a small amount of heat, should be introduced into the objective, or a change in temperature of the objective should have no influence or only little influence on the position of the focal plane.
Color-corrected F-theta objectives for laser applications are known from the conventional art for separate wavelengths, for example for 808 nm and 940 nm. These objectives are corrected for two or more wavelengths, but only for separate monochromatic emission lines.
In addition, color-corrected F-theta objectives which are designed for a particular monochromatic emission line of a processing laser and, for this purpose, have a corrected wavelength range for observing the working area are known.
An objective of this type is known from patent application EP 1 477 258 A1. The F-theta objective disclosed herein is a component part of an apparatus for local thermal treatment, with which point-accurate thermal treatment together with integrated monitoring of the working area is intended to be made possible. For sharp imaging of the working area, the objective must be corrected, in addition to for the wavelength of the processing laser, also for the wavelength range of the illumination radiation.
The F-theta objective disclosed herein comprises three elements, two of which elements have a collecting effect and one element has a dispersing effect, wherein one of the collecting elements is a lens component for achieving the necessary imaging quality. By suitably combining the glass types of the elements, in respect of which no further information is provided, the color correction for the corresponding wavelength ranges is intended to be achieved.
As a processing laser, examples mentioned are, as a solid-state laser, a Nd-YAG laser, fiber laser, disc laser or semiconductor laser. Such lasers have one or more monochromatic emission lines for which the F-theta objective is corrected.